How to Authenticate Higgsfield CLI via OAuth on a Headless Linux Server

A developer setting up the Higgsfield CLI on a headless server encountered an OAuth 2.0 PKCE authentication failure caused by a fundamental localhost mismatch between the server's HTTP callback listener and the laptop's browser. Because OAuth's loopback flow assumes the browser and listener share the same machine, the identity provider Clerk redirected the authentication callback to the laptop's localhost, where nothing was listening. The intended fix — an SSH port-forward tunnel — was complicated by the fact that Clerk only accepts pre-registered redirect URIs, meaning users cannot freely choose a port and must match one the app's developer whitelisted in advance. Dropping the custom port flag allows the CLI to use its registered default, but if that port is already occupied on the server, the CLI silently falls back to another registered port, potentially breaking the tunnel alignment. The core issue is not a bug but a deliberate security constraint: the redirect URI allowlist exists to prevent malicious actors from hijacking authenticated callbacks.